SolarCity, San Mateo

SolarCity has found that when you make a solar power system as free as the sunshine that fuels it, people notice. By removing the biggest roadblock for most people—installation of the panels is free, so you only pay for the power they generate—SolarCity estimates that it converts a new person to solar energy every three minutes. Walmart, eBay, and Intel have also taken notice; they're a few of SolarCity's business clients. Fortune recently ranked SolarCity's founders No. 3 on its list of The World's Top 25 Eco-Innovators (its chairman, Elon Musk, was No. 1), and the Stevie Awards presented the company with a gold award for its customer service.

The SolarCity team fields everything from customizing the design of each energy system to securing permits to jingling their keys until the sun looks their way. All its customers need to do is provide a roof (typically the one that came with their home) and flip the switch. The ease of use extends through the lifetime of the contract. Energy rates are predictable and immune to volatile market rates. SolarCity continuously monitors your system to ensure that it's online. All repairs are covered at no added cost if anything goes wrong.

Besides lowering energy bills, solar power has a major positive impact on the environment. The average solar-powered three-bedroom home offsets the carbon dioxide equivalent of driving a car 125,000 miles, according to data collected by the U.S. Environmental Protection Agency.